Patent GB-874261-A (GB)
Title: Improvements in co-operative collision data exchange systems for ships and aircraft
Inventor: VIELLE JOHN
IPC: G01S13/937
Filed: 1956-10-24
Abstract: 874,261. Electric correspondence control systems. AVEL CORPORATION GENEVA. Oct. 7, 1957 [Oct. 24, 1956 (2); June 6, 1957], Nos. 32473/56,[32474/56 and 18034/57. Class 40 (1). [Also in Group XL (c)] Relates to a system for the exchange of data between aircraft or ships for the prevention of collisions. Fig. 1 shows apparatus for use in aircraft, the barometric pressure, which is used as a measure of height, being measured at 1, 2, and applied to a " frequency unit " 4 which correspondingly varies the frequency of a ratio transmitter 3 whose signals are transmitted via an aerial 6. If the transmitter should fail, a warning light 8 operates. The course of the aircraft as derived from a compass 9 is coded at 10, the coder 10 keying the transmitter to provide pulses at a random repetition frequency whose length is dependent upon course. A receiver 11 is provided with directional and omni-directional aerials 12, 13, whereby the bearing of incoming transmissions is derived in known manner and displayed at 14. When a transmission is received this indicates that an adjacent aircraft with similar equipment is at approximately the same height since the frequencies accepted by the receiver 11 are determined by the frequency transmitted from the transmitter 3, and an indication is given by a warning light 15. The approximate range of an adjacent aircraft is determined by the strength of the received signals and indicated on a meter 16. A synchronizer 17 and switch 18 interrupt the receiver 11 during operation of the transmitter 3 and control the transmitter to bring the pulse repetition frequencies of the signals from the two aircraft into agreement. The output from the receiver 11 consists of an audio-frequency note interrupted by pulses of a length dependent upon the course of the transmitting aircraft, the audio note being passed to a height comparator 20 together with signals from the unit 4 so that the relative height of the other aircraft is presented on a meter 21, and the pulses being applied to a course comparator together with the signals from 10 to provide an indication on a meter 23 of the relative course of the other aircraft. In order to give a warning that he is about to change height the pilot may apply a varying sawtooth bias to the frequency unit 4 through a unit 24, and to discover whether any aircraft are at heights other than his own he may operate a search unit 25 which controls the receiver 11. It is stated that the receiving apparatus may be utilized in a ground station, and the transmitting apparatus may be used as a ground beacon to show the height of an obstruction or the minimum height in a flight zone. Some aircraft may be fitted with a transmitter only, operating on a fixed pulse repetition frequency. Fig. 2 shows the frequency unit 4, the height comparator 20, and the receiver 11. The unit 2 generates signals in the band 60 to 100 kc/s. representing heights between sea level and 40,000 feet. one cycle being equivalent to one foot. The unit 4 includes a crystal-controlled oscillator 30 operating on a frequency which is 60 kc/s. below the nominal carrier frequency of the transmitter 3 this frequency being mixed at 31 with the frequency derived from the unit 2 to provide an output occupying the 40 kc/s. band above the nominal carrier frequency, the lower sideband being suppressed, and this output controls the transmitter 3. The output from 31 is also applied to a mixer 33 together with signals from a crystal-controlled intermediatefrequency oscillator 32 to provide an output which is the transmitter frequency plus the intermediate frequency which is passed via an I.F. rejector 34 to a detector 35 having a onekilocycle bandwidth. The signals from the aerial 13 together with the output from a crystalcontrolled oscillator 36 operating at a frequency 500 c.p.s. below that of the oscillator 32 are combined at 35 and an audio output is derived which depends upon the difference in height between the two aircraft and varies between 0 and 1 kc/s. In one form of height comparator 20, Fig. 2, a 500 c.p.s. oscillation is produced at 37, which is controlled by the oscillators 32 and 36 and is fed to a differential frequency meter 38 together with the audio output from 35, the result being shown on a centre zero meter 21. Alternatively the output from the detector 35 may be fed to the primary of a transformer which has two secondary windings, Fig. 3 (not shown), one secondary winding being connected through a capacitor to a bridge rectifier circuit and the other winding through an inductor to a further bridge rectifier circuit, the outputs from the two rectifier circuits being combined in opposition in a centre zero.meter. The synchronizer 17, Fig. 4, comprises a low-frequency noise generator 60 which with an amplifier 61 incorporating a low-pass filter forms a random frequency generator for controlling a flip-flop 63 via a gate 62. Under normal operating conditions the flip-flop 63 operates the bi-stable switch 18 to switch on the transmitter 3 for the length of time determined by the course coder 10 and at the end of the course pulse the transmitter operates the switch 18 to energize the receiver 11, a further pulse from the multivibrator repeating the cycle. When a signal is received by the receiver 11 the a.g.c. line is energized to close the gate 62 and stop the random operation of the flip-flop 63. The incoming pulses from the receiver 11 are also differentiated and clipped at 64 whereby at the end of a received pulse the flip-flop is restarted and switch 18 operated to start the transmitter 3. In this way the two transmitters are caused to operate alternately each starting as soon as the other has switched off, If a third aircraft should come into the vicinity at this time its transmitter cannot come into synchronism and a warning light 19, Fig. 1, is operated. It is stated that by arranging that the flip-flop is switched by alternate pulses from the differentiator 64, alternate pulses may be used to transmit speed indications. In the course coder 10 and comparator 22, Fig. 5, the compass or gyro repeater operates a wiper 70 co-operating with a standard resistance 71 whereby a voltage dependent on the course is passed to a bridge network comprising standard resistances 72, 73, and the operating coil 74 of a device combining the functions of a voltmeter and a uniselector. The coil 74 is energized via a switch 75 which is closed only during the reception period, and thus during this period the pointer 76 of the coder 10 is moved over a standard resistance 77 to take up a position dependent upon the course. During the transmission period the switch 75 is opened and a train of standard pulses on lead 78 is fed to the operating coil 79 to step the uniselector and its pointer 76 back to zero and operate a limit switch 80 to stop the transmission. In this way the duration of the transmitted pulse is determined by the time taken for the pointer 76 to travel back to zero. The comparator comprises a uniselector having an operating coil 81 controlled by the same standard pulses appearing on lead 82. The output from the receiver 11 is differentiated at 83 whereby positive-going and negative-going pulses are derived to respectively open and close a gate 84 at the beginning and end of the main pulse. Thus the position of the pointer 85 on a standard resistance 86 is determined by the length of the received pulses and provides a voltage which together with the voltage derived from the wiper 70 is applied to a centre zero meter to provide a relative indication. In a modified system, Fig. 6 (not shown), instead of the transmitter frequency being varied in accordance with height, the carrier is modulated by an audio-frequency and both sidebands are transmitted. An additional transmitter and receiver may be provided for determining the bearing of the transmitting aircraft. As applied to ships, Fig. 8 (not shown), the course of the ship controls the transmitted frequency and the speed of the ship controls the pulse length, or an audio note may be transmitted which is characteristic of the course, Fig. 9 (not shown).